1. Field of the Invention
The present invention relates to a medical communication system and method for communicating between an implanted device and another device in the system.
2. Description of the Prior Art
In RF coupled systems, which are perhaps the most commonly employed communication systems in modern implantable device systems, information is transferred from a transmitting coil to a receiving coil with a radio-frequency carrier signal. The carrier signal is modulated with the data that is to be transmitted using an appropriate modulation scheme, such as phase shift keying (PSK), frequency shift keying (FSK), or pulse position modulation (PPM), among numerous others. The modulated carrier induces a voltage in the receiving coil that tracks the modulated carrier signal. This received signal is then demodulated in order to recover the transmitted data. Because the stainless steel or titanium can commonly used to hermetically enclose an implanted device acts as a low-pass filter for the transmitted RF signals, attenuation increases as frequency is increased. Devices currently on the market have a maximum frequency of less than 200-kHz. Also, the transmitting range has been limited to 2 to 3 inches or so.
Depending upon the type of modulation and demodulation used in an RF communication system, the data or bit rate cannot exceed a predetermined fraction of the carrier frequency; otherwise, the ability to reliably distinguish between modulation representing a digital (binary) xe2x80x9c1xe2x80x9d from a digital xe2x80x9c0xe2x80x9d is compromised. Schemes are known which encode digital data to transmit more data per unittime and reduce current drain in the implanted device. However, at very high data transmission rates, the current drain would be very high.
RF communication programming units typically interface with the implanted device through the use of a programming head or programming paddle, a handheld unit adapted to be placed on the patient""s body over the implant site of the patient""s implanted device. In some cases, a magnet in the programming head effects reed switch closure in the implanted device to initiate a communication session (this is a safeguard against accidental programming of the device; otherwise, reed switch closure has little meaning as far as communication of information). Thereafter, uplink and downlink communication takes place between the implanted device""s transmitter and receiver and a receiver and transmitter disposed within the programming head.
A newly proposed standard for Medical Implant Communications Service, MICS, states that a number of radio communication channels within a certain frequency range can be used to establish a communication link between an implanted device and an external unit, or between implanted devices. According to the standard one communication link, i.e. communication between two devices, is not allowed to use more than one channel at a time. If a channel becomes unusable of some reason the system can switch to another of the specified channels. Before a new channel can be accessed, the channel shall be monitored in a manner described by the standard in order to avoid collisions.
To avoid accessing a channel in use, a MICS system shall, according to the standard, monitor the channel within the frequency range allocated for MICS information transmission before attempting to establish contact. The earmarked frequency range for communication is divided into N channels. The standard states that a channel shall be monitored for a period of at least 10 ms within 5 seconds prior to access, to ensure it is not occupied. In a noisy environment the channel in use can become inaccessible and a rather frequent channel switching can be necessary due to circumstances beyond the control of the operator.
If a search fails on one channel, e.g. due to too high noise level or if the channel is already in use, a 10 ms search period must be started to monitor a new channel and the procedure must be repeated until a noise-free channel is found. Repetitive searches might result in several 10 ms search periods before a noise free channel is found which lowers the transmission stability of the communication link.
Furthermore, the standard prescribes a procedure how to investigate a channel before accessing it. In short a frequency monitoring is performed by incorporating a mechanism in a medical implant transmitter for monitoring the channel or channels that the MICS system devices intend to occupy. The monitoring system antenna shall be the antenna normally used by the transmitter for a communications session. Before a medical implant transmitter initiates a MICS communication session, the following access criteria must be met:
(1) The monitoring system bandwidth measured at its 20 dB down points must be equal to or greater than the emission bandwidth of the intended transmission.
(2) Within 5 seconds prior to initiating a communications session, circuitry associated with a medical implant transmitter must monitor the channel or channels the MICS system devices intend to occupy for a minimum of 10 milliseconds per channel. Before transmitting on an alternate channel, the channel must be monitored for a period of at least 10 milliseconds.
A similar way of detecting carrier frequencies is also included in a standard draft version from European Telecommunication Standard Institute (ETSI). The European standard covers radio equipment in the frequency range 402 MHz to 405 MHz for Ultra Low Power Active Medical Implants and Accessories. Within this frequency range the maximum permitted emission bandwidth for each channel is set to 300 kHz, i.e. 10 channels side by side starting from 402 MHz.
U.S. Pat. No. 6,150,951 relates to a medical telemetry system with wireless and physical communication channels. The system includes an apparatus for monitoring a transmission activity in a pre-given channel range for determining possible channels in use, so that the transmission channel is assigned to the transmitter in accordance with the determined channels in use. Before a transmitter will be used for transmission purposes in combination with a receiver, the receiver monitors the xe2x80x9con air activityxe2x80x9d in its environment for any transmission activity in a certain channel range assigned to the receiver. This phase is called the xe2x80x9cscanning phasexe2x80x9d. The receiver thus determines which channels are in use, e.g., by any other transmitter or by other functional units. The receiver may e.g. include a synthesizer receiver unit for stepping through a predefined channel range and for measuring the received signal strength on each of the channels. When the received signal strength of a certain channel exceeds a certain predefined value, the receiver will treat this channel as being in use. In this known device the transmitter is designated a specific channel during the scanning phase that it then uses during operation. One drawback with this system is that it is unable to handle a situation when e.g. noise disturbs the used specific channel during operation. The reason is that the scanning phase is performed before the transmitter will be used for transmission purposes.
An object of the present invention is to provide a technique for communication in a medical system having at least one implanted device that minimizes the time spent to decide which new channel to be used once the currently used channel becomes unusable in order to recognize an available channel for a fast channel switchover.
The above object is achieved in accordance with the principles of the present invention in a medical system having two units, at least one of the units being implanted in a subject, and in a method for communicating between two such units in a medical system, wherein a single channel, from among a number of communication channels, is used to establish a communication link between the two units during a communication time interval, the channel which is in use being an active channel, and the other channels being passive channels, and wherein the passive channels are monitored with regard to their availability for communication during the communication interval, and the result of this monitoring is stored in a register table having respective registers for the communication channels, and wherein the active channel is also monitored during the communication time interval and if the active channel becomes unsuitable for establishing said communication link, an automatic switchover is made to an available passive channel, which immediately then becomes the active channel.
Thus, instead of interrupting the transmission once the current channel is discarded and wasting time during the one or several 10 ms search periods the new channel(s) being monitored the monitoring process runs continuously and concurrently with the normal transmission process.